X-linked recessive mental retardation has been estimated to cause 5-36 percent of all mental retardation. Among both institutionalized and non-institutionalized retarded children there is an excess of approximately 50 percent males. In 1969, the present investigator reported a family with X-linked mental retardation in which all retarded males had an unusual X chromosome characterized by a secondary constriction or satellite-like appearance at the end of the long arm of the X. In both males and females the marker X was demonstrated in only a minority of cells. In most laboratories the culture conditions currently used do not permit its detection or at best only variable detection in a small proportion of cells. In spite of this, however, 40 families are now known with the same marker chromosome and mental retardation in the males having the chromosome. The present project is designed to optimize the conditions for detection of the marker X by a variety of alterations in the culture conditions, to further study the nature of the lesion by the use of a number of new cytogenetic technics and finally to estimate the frequency of the marker X chromosome. In this study the Marker X has been identified in 4 members of one family and an amniotic fluid culture is currently being studied under special culture conditions to determine its presence or absence. Optimal culture conditions for detection of the marker X, to date, have included 2 percent fetal calf serum, low thymidine and folic acid concentrations and Media 199 with Earle's salts. The significance of this work lies in improved detection of a preventable form of mental retardation and in improved understanding of what is probably a new type of chromosomal abnormality. Finally, it is likely at least as frequent as XYY or XXY and in contrast to these abnormalities appears to be associated with mental retardation in all cases. Thus, it may be comparable to or even more frequent than Down Syndrome as a cause of mental retardation.